Data encoding and decoding apparatus, method and storage medium

ABSTRACT

A video data decoding apparatus is configured to detect a control flag associated with at least a part of an encoded image for decoding, in which in a lossless mode of operation, a first control flag state enables sample-based angular intra-prediction but disables edge filtering of prediction samples, and a second control flag state disables sample-based angular intra prediction but enables edge filtering of prediction samples; and in a lossy mode of operation, the first control flag state enables residual differential pulse code modulation coding and enables edge filtering of prediction samples, and the second control flag state disables residual differential pulse code modulation coding but enables edge filtering of prediction samples.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 14/315,498 filed Jun. 26, 2014, the entire content of which isherein incorporated by reference in its entirety for all purposes.

FIELD OF THE DISCLOSURE

This disclosure relates to data encoding and decoding.

DESCRIPTION OF THE RELATED ART

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentdisclosure.

There are several video data compression and decompression systems whichinvolve transforming video data into a frequency domain representation,quantising the frequency domain coefficients and then applying some formof entropy encoding to the quantised coefficients.

The transformation into the spatial frequency domain at the encoder sidecorresponds to an inverse transformation at the decoder side. Exampletransformations include the so-called discrete cosine transformation(DCT) and the so-called discrete sine transformation (DST). In someexamples the transformations are carried out by matrix-multiplying anarray of input samples (derived from the video data to be coded) by amatrix of transformation coefficients to generate frequency-transformeddata. Frequency-transformed data is converted back to sample data, fromwhich output video data can be derived, by matrix-multiplying an arrayof the frequency-transformed data by a matrix of inverse-transformationcoefficients.

SUMMARY

An aspect of this disclosure is defined by claim 1.

Further respective aspects and features are defined in the appendedclaims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, but not restrictiveof, the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description ofembodiments, when considered in connection with the accompanyingdrawings, wherein:

FIG. 1 schematically illustrates an audio/video (A/V) data transmissionand reception system using video data compression and decompression;

FIG. 2 schematically illustrates a video display system using video datadecompression:

FIG. 3 schematically illustrates an audio/video storage system usingvideo data compression and decompression;

FIG. 4a schematically illustrates a video camera using video datacompression;

FIG. 4b schematically illustrates an example video camera in moredetail;

FIG. 4c schematically illustrates another example video camera;

FIGS. 4d and 4e schematically illustrate date carriers;

FIG. 5 provides a schematic overview of a video data compression anddecompression apparatus;

FIG. 6 schematically illustrates the generation of predicted images;

FIG. 7 schematically illustrates a largest coding unit (LCU):

FIG. 8 schematically illustrates a set of four coding units (CU);

FIGS. 9 and 10 schematically illustrate the coding units of FIG. 8sub-divided into smaller coding units;

FIG. 11 schematically illustrates an array of prediction units (PU);

FIG. 12 schematically illustrates an array of transform units (TU);

FIG. 13 schematically illustrates an array of samples;

FIG. 14 schematically illustrates an array of frequency-separatedcoefficients;

FIG. 15 schematically illustrates part of an encoder;

FIGS. 16 and 17 schematically illustrate an RDPCM operation;

FIG. 18 is a schematic flowchart illustrating operations of a encoder;

FIG. 19 schematically illustrates a flag;

FIG. 20 is a schematic flowchart illustrating operations of a decoder;

FIG. 21 schematically illustrates a part of a decoder; and

FIG. 22 schematically illustrates a part of an encoder.

DESCRIPTION OF THE EMBODIMENTS

Referring now to the drawings, FIGS. 1-4 e are provided to giveschematic illustrations of apparatus or systems making use of thecompression and/or decompression apparatus to be described below inconnection with embodiments.

All of the data compression and/or decompression apparatus is to bedescribed below may be implemented in hardware, in software running on ageneral-purpose data processing apparatus such as a general-purposecomputer, as programmable hardware such as an application specificintegrated circuit (ASIC) or field programmable gate array (FPGA) or ascombinations of these. In cases where the embodiments are implemented bysoftware and/or firmware, it will be appreciated that such softwareand/or firmware, and non-transitory machine-readable data storage mediaby which such software and/or firmware are stored or otherwise provided,are considered as embodiments.

FIG. 1 schematically illustrates an audio/video data transmission andreception system using video data compression and decompression.

An input audio/video signal 10 is supplied to a video data compressionapparatus 20 which compresses at least the video component of theaudio/video signal 10 for transmission along a transmission route 30such as a cable, an optical fibre, a wireless link or the like. Thecompressed signal is processed by a decompression apparatus 40 toprovide an output audio/video signal 50. For the return path, acompression apparatus 60 compresses an audio/video signal fortransmission along the transmission route 30 to a decompressionapparatus 70.

The compression apparatus 20 and decompression apparatus 70 cantherefore form one node of a transmission link. The decompressionapparatus 40 and decompression apparatus 60 can form another node of thetransmission link. Of course, in instances where the transmission linkis uni-directional, only one of the nodes would require a compressionapparatus and the other node would only require a decompressionapparatus.

FIG. 2 schematically illustrates a video display system using video datadecompression. In particular, a compressed audio/video signal 100 isprocessed by a decompression apparatus 110 to provide a decompressedsignal which can be displayed on a display 120. The decompressionapparatus 110 could be implemented as an integral part of the display120, for example being provided within the same casing as the displaydevice. Alternatively, the decompression apparatus 110 might be providedas (for example) a so-called set top box (STB), noting that theexpression “set-top” does not imply a requirement for the box to besited in any particular orientation or position with respect to thedisplay 120; it is simply a term used in the art to indicate a devicewhich is connectable to a display as a peripheral device.

FIG. 3 schematically illustrates an audio/video storage system usingvideo data compression and decompression. An input audio/video signal130 is supplied to a compression apparatus 140 which generates acompressed signal for storing by a store device 150 such as a magneticdisk device, an optical disk device, a magnetic tape device, a solidstate storage device such as a semiconductor memory or other storagedevice. For replay, compressed data is read from the store device 150and passed to a decompression apparatus 160 for decompression to providean output audio/video signal 170.

It will be appreciated that the compressed or encoded signal, and astorage medium or data carrier storing that signal, are considered asembodiments. Reference is made to FIGS. 4d and 4e described below.

FIG. 4a schematically illustrates a video camera using video datacompression. In FIG. 4a , and image capture device 180, such as a chargecoupled device (COD) image sensor and associated control and read-outelectronics, generates a video signal which is passed to a compressionapparatus 190. A microphone (or plural microphones) 200 generates anaudio signal to be passed to the compression apparatus 190. Thecompression apparatus 190 generates a compressed audio/video signal 210to be stored and/or transmitted (shown generically as a schematic stage220).

The techniques to be described below relate primarily to video datacompression. It will be appreciated that many existing techniques may beused for audio data compression in conjunction with the video datacompression techniques which will be described, to generate a compressedaudio/video signal. Accordingly, a separate discussion of audio datacompression will not be provided. It will also be appreciated that thedata rate associated with video data, in particular broadcast qualityvideo data, is generally very much higher than the data rate associatedwith audio data (whether compressed or uncompressed). It will thereforebe appreciated that uncompressed audio data could accompany compressedvideo data to form a compressed audio/video signal. It will further beappreciated that although the present examples (shown in FIGS. 1-4 e)relate to audio/video data, the techniques to be described below canfind use in a system which simply deals with (that is to say,compresses, decompresses, stores, displays and/or transmits) video data.That is to say, the embodiments can apply to video data compressionwithout necessarily having any associated audio data handling at all.

FIG. 4b schematically illustrates an example video camera apparatus 183in more detail. Those features numbered in common with FIG. 4a will notbe described further. FIG. 4b is an example of the camera of FIG. 4a (inthe case that the unit 220 of FIG. 4a provides a storage capability) inwhich the compressed data are first buffered by a buffer 221 and thenstored in a storage medium 222 such as a magnetic disk, an optical disk,flash memory, a so-called solid-state disk drive (SSD) or the like. Notethat the arrangement of FIG. 4b can be implemented as a single(physical) unit 182.

FIG. 4c schematically illustrates another example video camera in which,in place of the storage arrangement of FIG. 4b , a network interface 223is provided in order to allow the compressed data to be transmitted toanother unit (not shown). The network interface 223 can also allow forincoming data to be received by the video camera, such as control data.Note that the arrangement of FIG. 4b can be implemented as a single(physical) unit 183.

FIGS. 4d and 4e schematically illustrate data carriers, for example foruse as the storage medium 222 and carrying compressed data which hasbeen compressed according to the compression techniques described in thepresent application. FIG. 4d shows a schematic example of a removablenon-volatile storage medium 225 implemented as solid state memory suchas flash memory. FIG. 4e shows a schematic example of a removablenon-volatile storage medium 226 implemented as a disk medium such as anoptical disk.

FIG. 5 provides a schematic overview of a video data compression anddecompression apparatus.

Successive images of an input video signal 300 are supplied to an adder310 and to an image predictor 320. The image predictor 320 will bedescribed below in more detail with reference to FIG. 6. The adder 310in fact performs a subtraction (negative addition) operation, in that itreceives the input video signal 300 on a “+” input and the output of theimage predictor 320 on a “−” input, so that the predicted image issubtracted from the input image. The result is to generate a so-calledresidual image signal 330 representing the difference between the actualand projected images.

One reason why a residual image signal is generated is as follows. Thedata coding techniques to be described, that is to say the techniqueswhich will be applied to the residual image signal, tends to work moreefficiently when there is less “energy” in the image to be encoded.Here, the term “efficiently” refers to the generation of a small amountof encoded data; for a particular image quality level, it is desirable(and considered “efficient”) to generate as little data as ispracticably possible. The reference to “energy” in the residual imagerelates to the amount of information contained in the residual image. Ifthe predicted image were to be identical to the real image, thedifference between the two (that is to say, the residual image) wouldcontain zero information (zero energy) and would be very easy to encodeinto a small amount of encoded data. In general, if the predictionprocess can be made to work reasonably well, the expectation is that theresidual image data will contain less information (less energy) than theinput image and so will be easier to encode into a small amount ofencoded data.

The residual image data 330 is supplied to a transform unit 340 whichgenerates a discrete cosine transform (DCT) representation of theresidual image data. The DCT technique itself is well known and will notbe described in detail here. There are however aspects of the techniquesused in the present apparatus which will be described in more detailbelow, in particular relating to the selection of different blocks ofdata to which the DCT operation is applied. These will be discussed withreference to FIGS. 7-12 below.

Note that in some embodiments, a discrete sine transform (DST) is usedinstead of a DCT. In other embodiments, no transform might be used. Thiscan be done selectively, so that the transform stage is, in effect,bypassed, for example under the control of a “transform skip” command ormode.

The output of the transform unit 340, which is to say, a set oftransform coefficients for each transformed block of image data, issupplied to a quantiser 350. Various quantisation techniques are knownin the field of video data compression, ranging from a simplemultiplication by a quantisation scaling factor through to theapplication of complicated lookup tables under the control of aquantisation parameter. The general aim is twofold. Firstly, thequantisation process reduces the number of possible values of thetransformed data. Secondly, the quantisation process can increase thelikelihood that values of the transformed data are zero. Both of thesecan make the entropy encoding process work more efficiently ingenerating small amounts of compressed video data.

A controller 345 controls the operation of the transform unit 340 andthe quantiser 350 (and their respective inverse units), according totechniques to be discussed further below. Note that the controller 345may also control other aspects of the operation of the apparatus of FIG.5.

A data scanning process is applied by a scan unit 360. The purpose ofthe scanning process is to reorder the quantised transformed data so asto gather as many as possible of the non-zero quantised transformedcoefficients together, and of course therefore to gather as many aspossible of the zero-valued coefficients together. These features canallow so-called run-length coding or similar techniques to be appliedefficiently. So, the scanning process involves selecting coefficientsfrom the quantised transformed data, and in particular from a block ofcoefficients corresponding to a block of image data which has beentransformed and quantised, according to a “scanning order” so that (a)all of the coefficients are selected once as part of the scan, and (b)the scan tends to provide the desired reordering. One example scanningorder which can tend to give useful results is a so-called zigzagscanning order.

The scanned coefficients are then passed to an entropy encoder (EE) 370.Again, various types of entropy encoding may be used. Two examples arevariants of the so-called CABAC (Context Adaptive Binary ArithmeticCoding) system and variants of the so-called CAVLC (Context AdaptiveVariable-Length Coding) system. In general terms, CABAC is considered toprovide a better efficiency, and in some studies has been shown toprovide a 10-20% reduction in the quantity of encoded output data for acomparable image quality compared to CAVLC. However, CAVLC is consideredto represent a much lower level of complexity (in terms of itsimplementation) than CABAC. Note that the scanning process and theentropy encoding process are shown as separate processes, but in factcan be combined or treated together. That is to say, the reading of datainto the entropy encoder can take place in the scan order. Correspondingconsiderations apply to the respective inverse processes.

The output of the entropy encoder 370, along with additional data, forexample defining the manner in which the predictor 320 generated thepredicted image, provides a compressed output video signal 380.

However, a return path is also provided because the operation of thepredictor 320 itself depends upon a decompressed version of thecompressed output data.

The reason for this feature is as follows. At the appropriate stage inthe decompression process a decompressed version of the residual data isgenerated. This decompressed residual data has to be added to apredicted image to generate an output image (because the originalresidual data was the difference between the input image and a predictedimage). In order that this process is comparable, as between thecompression side and the decompression side, the predicted imagesgenerated by the predictor 320 should be the same during the compressionprocess and during the decompression process. Of course, atdecompression, the apparatus does not have access to the original inputimages, but only to the decompressed images. Therefore, at compression,the predictor 320 bases its prediction (at least, for inter-imageencoding) on decompressed versions of the compressed images.

The entropy encoding process carried out by the entropy encoder 370 isconsidered to be “lossless”, which is to say that it can be reversed toarrive at exactly the same data which was first supplied to the entropyencoder 370. So, the return path can be implemented before the entropyencoding stage. Indeed, the scanning process carried out by the scanunit 360 is also considered lossless, but in the present embodiment thereturn path 390 is from the output of the quantiser 350 to the input ofa complimentary inverse quantiser 420.

In general terms, an entropy decoder 410, the reverse scan unit 400, aninverse quantiser 420 and an inverse transform unit 430 provide therespective inverse functions of the entropy encoder 370, the scan unit360, the quantiser 350 and the transform unit 340. For now, thediscussion will continue through the compression process; the process todecompress an input compressed video signal corresponds to the returnpath of the compression process.

In the compression process, the scanned coefficients are passed by thereturn path 390 from the quantiser 350 to the inverse quantiser 420which carries out the inverse operation of the scan unit 360. An inversequantisation and inverse transformation process are carried out by theunits 420, 430 to generate a compressed-decompressed residual imagesignal 440.

The image signal 440 is added, at an adder 450, to the output of thepredictor 320 to generate a reconstructed output image 460. This formsone input to the image predictor 320.

Turning now to the process applied to a received compressed video signal470, the signal is supplied to the entropy decoder 410 and from there tothe chain of the reverse scan unit 400, the inverse quantiser 420 andthe inverse transform unit 430 before being added to the output of theimage predictor 320 by the adder 450. In straightforward terms, theoutput 460 of the adder 450 forms the output decompressed video signal480. In practice, further filtering may be applied before the signal isoutput.

FIG. 6 schematically illustrates the generation of predicted images, andin particular the operation of the image predictor 320.

There are two basic modes of prediction: so-called intra-imageprediction and so-called inter-image, or motion-compensated (MC),prediction.

Intra-image prediction bases a prediction of the content of a block ofthe image on data from within the same image. This corresponds toso-called I-frame encoding in other video compression techniques. Incontrast to I-frame encoding, where the whole image is intra-encoded, inthe present embodiments the choice between intra- and inter-encoding canbe made on a block-by-block basis, though in other embodiments thechoice is still made on an image-by-image basis.

Motion-compensated prediction makes use of motion information whichattempts to define the source, in another adjacent or nearby image, ofimage detail to be encoded in the current image. Accordingly, in anideal example, the contents of a block of image data in the predictedimage can be encoded very simply as a reference (a motion vector)pointing to a corresponding block at the same or a slightly differentposition in an adjacent image.

Returning to FIG. 6, two image prediction arrangements (corresponding tointra- and inter-image prediction) are shown, the results of which areselected by a multiplexer 500 under the control of a mode signal 510 soas to provide blocks of the predicted image for supply to the adders 310and 450. The choice is made in dependence upon which selection gives thelowest “energy” (which, as discussed above, may be considered asinformation content requiring encoding), and the choice is signalled tothe encoder within the encoded output data stream. Image energy, in thiscontext, can be detected, for example, by carrying out a trialsubtraction of an area of the two versions of the predicted image fromthe input image, squaring each pixel value of the difference image,summing the squared values, and identifying which of the two versionsgives rise to the lower mean squared value of the difference imagerelating to that image area.

The actual prediction, in the intra-encoding system, is made on thebasis of image blocks received as part of the signal 460, which is tosay, the prediction is based upon encoded-decoded image blocks in orderthat exactly the same prediction can be made at a decompressionapparatus. However, data can be derived from the input video signal 300by an intra-mode selector 520 to control the operation of theintra-image predictor 530.

For inter-image prediction, a motion compensated (MC) predictor 540 usesmotion information such as motion vectors derived by a motion estimator550 from the input video signal 300. Those motion vectors are applied toa processed version of the reconstructed image 460 by the motioncompensated predictor 540 to generate blocks of the inter-imageprediction.

The processing applied to the signal 460 will now be described. Firstly,the signal is filtered by a filter unit 560. This involves selectivelyapplying a “deblocking” filter to remove or at least tend to reduce theeffects of the block-based processing carried out by the transform unit340 and subsequent operations. Also, an adaptive loop filter is appliedusing coefficients derived by processing the reconstructed signal 460and the input video signal 300. The adaptive loop filter is a type offilter which, using known techniques, applies adaptive filtercoefficients to the data to be filtered. That is to say, the filtercoefficients can vary in dependence upon various factors. Data definingwhich filter coefficients to use is included as part of the encodedoutput data stream.

The filtered output from the filter unit 560 in fact forms the outputvideo signal 480. It is also buffered in one or more image stores 570;the storage of successive images is a requirement of motion compensatedprediction processing, and in particular the generation of motionvectors. To save on storage requirements, the stored images in the imagestores 570 may be held in a compressed form and then decompressed foruse in generating motion vectors. For this particular purpose, any knowncompression decompression system may be used. The stored images arepassed to an interpolation filter 580 which generates a higherresolution version of the stored images; in this example, intermediatesamples (sub-samples) are generated such that the resolution of theinterpolated image is output by the interpolation filter 580 is 8 times(in each dimension) that of the images stored in the image stores 570.The interpolated images are passed as an input to the motion estimator550 and also to the motion compensated predictor 540.

In embodiments, a further optional stage is provided, which is tomultiply the data values of the input video signal by a factor of fourusing a multiplier 600 (effectively just shifting the data values leftby two bits), and to apply a corresponding divide operation (shift rightby two bits) at the output of the apparatus using a divider orright-shifter 610. So, the shifting left and shifting right changes thedata purely for the internal operation of the apparatus. This measurecan provide for higher calculation accuracy within the apparatus, as theeffect of any data rounding errors is reduced.

The way in which an image is partitioned for compression processing willnow be described. At a basic level, and image to be compressed isconsidered as an array of blocks of samples. For the purposes of thepresent discussion, the largest such block under consideration is aso-called largest coding unit (LCU) 700 (FIG. 7), which represents asquare array of 64×64 samples. Here, the discussion relates to luminancesamples. Depending on the chrominance mode, such as 4:4:4, 4:2:2, 4:2:0or 4:4:4:4 (GBR plus key data), there will be differing numbers ofcorresponding chrominance samples corresponding to the luminance block.

Three basic types of blocks will be described: coding units, predictionunits and transform units. In general terms, the recursive subdividingof the LCUs allows an input picture to be partitioned in such a way thatboth the block sizes and the block coding parameters (such as predictionor residual coding modes) can be set according to the specificcharacteristics of the image to be encoded.

The LCU may be subdivided into so-called coding units (CU). Coding unitsare always square and have a size between 8×8 samples and the full sizeof the LCU 700. The coding units can be arranged as a kind of treestructure, so that a first subdivision may take place as shown in FIG.8, giving coding units 710 of 32×32 samples; subsequent subdivisions maythen take place on a selective basis so as to give some coding units 720of 16×16 samples (FIG. 9) and potentially some coding units 730 of 8×8samples (FIG. 10). Overall, this process can provide a content-adaptingcoding tree structure of CU blocks, each of which may be as large as theLCU or as small as 8×8 samples. Encoding of the output video data takesplace on the basis of the coding unit structure.

FIG. 11 schematically illustrates an array of prediction units (PU). Aprediction unit is a basic unit for carrying information relating to theimage prediction processes, or in other words the additional data addedto the entropy encoded residual image data to form the output videosignal from the apparatus of FIG. 5. In general, prediction units arenot restricted to being square in shape. They can take other shapes, inparticular rectangular shapes forming half of one of the square codingunits, as long as the coding unit is greater than the minimum (8×8)size. The aim is to allow the boundary of adjacent prediction units tomatch (as closely as possible) the boundary of real objects in thepicture, so that different prediction parameters can be applied todifferent real objects. Each coding unit may contain one or moreprediction units.

FIG. 12 schematically illustrates an array of transform units (TU). Atransform unit is a basic unit of the transform and quantisationprocess. Transform units are always square and can take a size from 4×4up to 32×32 samples. Each coding unit can contain one or more transformunits. The acronym SDIP-P in FIG. 12 signifies a so-called shortdistance intra-prediction partition. In this arrangement only onedimensional transforms are used, so a 4×N block is passed through Ntransforms with input data to the transforms being based upon thepreviously decoded neighbouring blocks and the previously decodedneighbouring lines within the current SDIP-P.

FIG. 13 schematically illustrates an array of samples. These in factrepresent samples of the residual image 330 discussed above, and aredrawn at their appropriate relative spatial positions, which is to saythat a sample 800 lies at the upper left corner of an array of 4×4adjacent samples, and a sample 810 lies at the bottom right corner ofsuch an array, with respect to positions of those samples within animage.

It will of course be appreciated that the 4×4 array of FIG. 13 is justan example; the techniques and attributes discussed here can apply toarrays of various different sizes such as 8×8, 16×16, 32×32 and so on.

The samples of FIG. 13 are processed by the transform unit 340 togenerate frequency-separated coefficients, FIG. 14 schematicallyillustrates an array of such frequency-separated coefficients. Here, theposition of a coefficient within the array represents the spatialfrequencies corresponding to that coefficient. By convention, aso-called DC coefficient 820 occupies the upper-left array position.Moving towards the right within the array of FIG. 14 indicates anincreasing horizontal frequency component, and moving towards the bottomof the array of FIG. 14 indicates an increasing vertical frequencycomponent. Note that although it is just a convention to represent thearray of coefficients in this manner (rather than, for example, havingthe DC coefficient in the bottom-right corner), the ordering of thecoefficients is technically relevant to other parts of the process. Onereason for this is schematically illustrated by a broken-line arrow 830in FIG. 14, which indicates that moving from the bottom-right to thetop-left positions within the array of FIG. 14 corresponds to agenerally increasing importance of the coefficients to imagereconstruction. That is to say, in order to reconstruct the array orblock of samples of FIG. 13, the most important one of the coefficientsof FIG. 14 is the DC coefficient 820, followed in terms of importance bythe lower horizontal and vertical frequency coefficients.

In general terms, this trend of importance can also correspond to atrend in terms of coefficient magnitude, in that the magnitude of the DCcoefficient can tend to be the largest within the set of coefficientsderived from a block of samples. FIG. 15 schematically illustrates sucha trend within the array of FIG. 14, in which smaller values tend to betowards the lower right of the array and larger values tend to betowards the upper left of the array. Of course, a specific individualarray of coefficients may differ from this general trend.

FIG. 15 schematically illustrates part of an encoder and represents amodification of the operation of the transform unit 340 of FIG. 5. Notethat a corresponding modification would be provided to the inversetransform unit 430.

A selector 900, which may in fact be implemented as part of thefunctionality of the controller 345, is provided to select (using aschematic switch 910) between three modes of operation, namely a mode inwhich a frequency transform is used (routing the data via the transformunit 340), a transform skip mode in which no transform is employed, anda so-called RDPCM (residual differential pulse code modulation) modeusing an RDPCM unit 342. Data which has been handled by any of the threeroutes is combined by an output multiplexer 920, Note that the selector900 may also control part of the operation of the entropy encoder 370;this feature will be discussed further below. Note also that thetransform operation can be omitted in a so-called trans-quant bypassmode, in which neither the transform nor the quantisation operations areapplied.

FIGS. 16 and 17 schematically illustrate an RDPCM operation. Theoperation is applied to a block of residual data 330, an example ofwhich is schematically illustrated in FIG. 16. In FIG. 16, an exampleblock of 4×4 samples is used, but the techniques are applicable to otherblock sizes. Each sample in the block is given a unique letter referencefrom A to P in order that the effect of the RDPCM process may beillustrated in FIG. 17.

RDPCM may be applied in a horizontal (left to right) sense or a vertical(top to bottom) sense. FIG. 17 schematically illustrates the horizontalRDPCM process, but it will be appreciated that the same techniques areapplicable to the vertical process.

Referring to FIG. 17, a left-hand column of samples (A, E, I, M) remainunchanged by the coding process. After that, each successive samplealong a row is encoded in terms of the difference between that sampleand a reconstructed version of the sample to its left. Here, the term“reconstructed” refers to the value of that sample after it has beenencoded and then decoded (so that both the encode and decode are workingwith respect to data available to the decoder). An asterisk (*) is usedto signify a reconstructed version of a sample so that, for example,B*represents a reconstructed version of the sample B. So, the sample Bis encoded by the value (B−A*), the sample K is encoded by the value(K−J*) and so on.

RDPCM coding can be used in intra-image-encoding orinter-image-encoding. In examples of the HEVC system, inintra-image-encoding, horizontal RDPCM is used in a horizontalprediction mode, which in turn determines the scan order as a reversevertical scan, and vertical RDPCM is used in a vertical prediction mode,which again in turn determines this can order as a reverse horizontalscan. In inter-image-encoding, either horizontal or vertical RDPCM maybe selected (for example, by the control at 345 in response tocalculation of a cost function) and the scan mode is always reversediagonal.

A similar technique to RDPCM is that of sample based angularintra-prediction (SAIP) which has been proposed, for example in US20130101036 A1 (the contents of which are incorporated herein byreference). In this technique a predicted sample is generated byreference to an interpolation or similar relationship to one or moreadjacent samples. The direction of the adjacent samples can bedetermined by the current prediction direction.

The selection of RDPCM, SAIP or other intra-prediction techniques, whichmay include as possible techniques a so-called intra-block-copy (IBC)encoding, is made at the encoder. IBC represents a hybrid of inter- andintra-encoding, such that a block is generated by copying another blockfrom another position within the same image, according to a displacementwhich may be considered to be similar in some respects to a motionvector. The encoder (for example, the controller 345) can carry out oneor more trial encodings (or at least trial derivations of the amount ofdata and/or the error rate and/or some other cost function) which wouldresult from encoding the image, or a particular block, using the varioustechniques. On the basis of an analysis of such a cost function, theencoder selects the appropriate encoding technique in respect of eachblock.

A proposal is that SAIP is used only in respect of a lossless mode ofoperation. That is not to say that every losslessly-encoded block mustuse SAIP, but rather than SAIP may not be used other than in a losslessmode of operation. Similarly, a proposal is that RDPCM is used only inrespect of a lossy mode of operation. Once again, that is not to saythat every lossy-encoded block must use RDPCM, but rather that RDPCM maynot be used other than in a lossy mode of operation.

Here, a lossy mode makes use of techniques such as quantisation whichdeliberately discard some of the information content of the originaldata, generally with the overall aim that the information content isdiscarded in such a way as not to be noticeable, or at least to have agenerally even effect across the images which provides as littleobjective disturbance to the decoder and subjective disturbance to theviewer as possible given the amount of information which is beingdiscarded. In a lossy mode, the reconstructed data at the decoder isintended to be a good approximation to, but not an exact reproductionof, the original data provided to the encoder. In contrast, in alossless mode, techniques such as quantisation are not used. The datareconstructed by the decoder is an exact replica of the data originallysupplied to the encoder. In the case of a data compression system suchas HEVC which is based around encoding residual differences from apredicted version of an image which in turn is based on reconstructedreference samples, it is noted that the reference samples used in alossless mode will be exact versions of the corresponding samples of theoriginally encoded images.

Various stages of sample filtering relevant to the use of RDPCM and SAIP(and indeed to other modes of intra operation) will now be discussed.

-   -   a) a so-called intra reference sample smoothing filter operates        on reference samples from which other samples will be derived in        intra-prediction. It is generally considered not be needed in a        lossless mode of operation, because the reference samples are        deemed to be perfect, having suffered no degradation or loss in        the encoding/decoding process. An encoder flag is provided to        allow the use of this filter to be enabled or disabled by        instruction of the encoder. If the intra reference sample        smoothing filter is in use, it may be further gated in its        operation by the prediction direction and block size, such that        it is used only for a subset of possible prediction directions        and/or a subset of possible block sizes.    -   b) an edge filter operates to filter block edges within data        from which intra-predicted samples will be derived, so as to        generally increase the extent to which the block edges are        smooth and continuous. This is used only in horizontal and        vertical prediction modes. In modes representing other        directions of prediction, this filter is not used, nor is it        used for chroma prediction or for larger block sizes.    -   c) a DC filter is used in DC mode (that is to say, not        horizontal, not vertical and not directional prediction). The DC        mode uses as inputs all of the reference samples around a        current block and takes an average, setting the predicted value        of each sample in the block to be the average value of those        reference samples. For predicted samples at the left and top        edges, a filter is applied to blend with the neighbouring        reference samples.

In connection with the filtering stage as discussed above, particularreference will now be made to the edge filter (b).

In particular, in embodiments of the present disclosure, a singleencoder flag to be referred to as the intra_RDPCM_enable flag, isprovided to indicate a mode of operation from the encoder to thedecoder. The interpretation of this flag at the decoder depends uponwhether the current mode of operation is lossy or lossless:

-   -   i. When the intra_RDPCM_enable flag is enabled in a lossless        mode, the decoder is instructed to use sample-based angular        intra-prediction (SAIP) but to disable the edge filter discussed        above.    -   ii. When the intra_RDPCM_enable flag is not enabled in a        lossless mode, the decoder is instructed not to use sample-based        angular intra-prediction but to enable use of the edge filter.    -   iii. When the intra_RDPCM_enable flag is enabled in a lossy        mode, the decoder is instructed to enable use of the edge filter        and to use RDPCM,    -   iv. When the intra_RDPCM_enable flag is not enabled in a lossy        mode, the decoder is instructed to use the edge filter but not        to use RDPCM.

Advantageously, embodiments of the present disclosure conveniently allowa single flag not only to control the use, by the decoder, of RDPCM, butalso to control the use of sample-based angular intra-prediction and theedge filter. Previously proposed systems used separate flags forenabling and disabling sample-based angular intra-prediction and forenabling and disabling RDPCM. Accordingly, the advantageous use of asingle flag is not only more convenient but also can reduce the dataoverhead involved in signalling between the encoder and decoder.

FIG. 18 is a schematic flowchart illustrating operations of an encoder,in order to generate or provide the intra_RDPCM_enable flag. Such anencoder may be implemented as, for example, the encoder shown in FIG. 5.The operations shown schematically in FIG. 18 may be implemented by, forexample, the controller 345.

At a step 950, the encoder detects whether its current mode of operationis a lossy mode. If so, control passes to a step 970. If not, controlpasses to a step 960. Having said this, the step 950 (and indeed thestep 960) are shown for the purposes of a schematic explanation, but inpractice, a real implementation of an encoder would be expected just toact in accordance with its current mode, which is to say by proceedingdirectly to either the step 970 or to a (lossless mode) step 985 independence upon its current mode of operation.

If the current mode is lossy, then at the step 970, the encoderdetermines whether or not to enable RDPCM operation in respect of acurrent block, slice, picture or stream. The encoder sets theintra_RDPCM_enable flag at a step 980 in response to this determination.

Turning to the step 960, corresponding to a lossless mode, at asubsequent step 985, the encoder determines whether to enablesample-based angular intra-prediction (SAIP) or to enable edgefiltering. The encoder sets the intra_RDPCM_enable flag at a step 990 inresponse to this determination.

The steps 970 and 985 can, as discussed above, involve the controller345 in controlling operation of the encoder to carry out one or moretrial encodings, or analysis relating to the amount of data and/oranother cost function which would apply if the data were encoded usingthat technique.

At a step 995, the encoder transmits the flag, for example as part ofthe data stream corresponding to the encoded video data.

Accordingly, FIG. 18 provides an example of a video data encoding methodcomprising: generating (the steps 980, 990) a control flag associatedwith at least a part of an encoded image, by: setting a first controlflag state in a lossless mode of operation to enable sample-basedangular intra-prediction but to disable edge filtering of predictionsamples; and setting a second control flag state in the lossless mode ofoperation to disable sample-based angular intra prediction but to enableedge filtering of prediction samples (both relating to the step 985);setting the first control flag state in a lossy mode of operation toenable residual differential pulse code modulation coding and to enableedge filtering of prediction samples; and setting the second controlflag state in the lossy mode of operation to disable residualdifferential pulse code modulation coding but to enable edge filteringof prediction samples in response to the second control flag state (bothrelating to the step 970).

As discussed above, the selections of modes of operation underlying thatrepresented by the flag may be chosen by the controller 345 based uponone or more trial encodings, part encodings or other analysis. The flagmay be incorporated within control data relating to a block, a slice, animage, a sequence or a stream. FIG. 19 provides an example 1060 of sucha flag. It will be understood that the exact polarity of such a flag(whether a particular flag value such as “1” represents the flag beingenabled or the flag being disabled, or even if more than one data bit isused to encode the flag) is not technically significant to the presentdisclosure. Nevertheless, in an example system, a data value of “1”indicates an enabled flag.

FIG. 20 is a schematic flowchart illustrating operations of a decoder,in response to receipt of the intra_RDPCM_enable flag. As discussed, adecoder may be implemented using the same components and techniques asthe decoding path of an encoder such as that shown in FIG. 5. Theoperations shown schematically in FIG. 20 may be implemented by, forexample, the controller 345.

At a step 1000, the decoder detects whether its current mode ofoperation is a lossy mode. If not, control passes to a step 1010 to bediscussed below. If however the mode is lossy, then, depending on thestate of the intra_RDPCM_enable flag, control passes either to a step1020 or to a step 1030. At the step 1020, if the intra_RDPCM_enable flagis enabled, the edge filter is enabled and RDPCM is also enabled. Ifnot, then at the step 1030 corresponding to the intra_RDPCM_enable flagnot being enabled, the edge filter is still enabled but RDPCM is notenabled.

Turning to the step 1010, corresponding to a lossless mode, depending onthe state of the intra_RDPCM_enable flag, control passes either to astep 1040 or to a step 1050. At the step 1040, if the intra_RDPCM_enableflag is enabled then sample-based angular intra-prediction is enabledbut edge filtering is not enabled. On the other hand, at the step 1050,if the intra_RDPCM_enable flag is not enabled, then sample-based angularintra-prediction is not enabled but edge filtering is enabled.

Accordingly, FIG. 20 provides an example of a video data decoding methodcomprising: detecting a control flag associated with at least a part ofan encoded image for decoding (at the input to the step 1000); in alossless mode of operation (the steps 1010 to 1050), enabling (1040)sample-based angular intra-prediction but disabling edge filtering ofprediction samples in response to a first control flag state; in thelossless mode of operation, disabling (1050) sample-based angular intraprediction but enabling edge filtering of prediction samples in responseto a second control flag state; in a lossy mode of operation (the steps1000-1030), enabling (1020) residual differential pulse code modulationcoding and enabling edge filtering of prediction samples in response tothe first control flag state; and in the lossy mode of operation,disabling (1030) residual differential pulse code modulation coding butenabling edge filtering of prediction samples in response to the secondcontrol flag state.

FIG. 21 schematically illustrates a part of a decoder (or indeed thedecoding path of an encoder). Video data for decoding is provided to aflag detector 1100 which detects a control flag present in or associatedwith the video data. In this example, the control flag is theintra_RDPCM_enable flag discussed above. The flag detector 1100 passesinformation to a controller (such as the controller 345 discussed above)indicative of the state (enabled or disabled) of the intra_RDPCM_enableflag. The controller 345 is also responsive to the current mode (lossyor lossless) of the encoder.

In response to the state of the control flag, the controller 345controls operation of an edge filter 1110, a sample-based angularintra-prediction decoder 1120, a residual differential pulse codemodulation decoder 1130 and one or more other decoders 1140 which,according to the control and enablement of the controller 345, act uponthe encoded data to generate output data 1150 which is then (again,according to the control and enablement of the controller 345) filteredby the edge filter 1110.

Note that it is not essential that a decoder can operate in both a lossyand a lossless mode, just that it is responsive to the control flag inaccordance with its mode of operation. Accordingly, it is not essentialthat the decoder contains both the SAIP decoder 1120 and the RDPCMdecoder 1130, as long as it responds to the control flag according tothe parameters set out above.

Accordingly, this arrangement provides an example of a video datadecoding apparatus operable to detect a control flag associated with atleast a part of an encoded image for decoding, in which: in a losslessmode of operation, a first control flag state enables sample-basedangular intra-prediction but disables edge filtering of predictionsamples, and a second control flag state disables sample-based angularintra prediction but enables edge filtering of prediction samples; andin a lossy mode of operation, the first control flag state enablesresidual differential pulse code modulation coding and enables edgefiltering of prediction samples, and the second control flag statedisables residual differential pulse code modulation coding but enablesedge filtering of prediction samples.

In embodiments, the apparatus comprises an edge filter 1110; and acontroller 345 responsive to the detection of the control flag to enableor disable the edge filter.

In embodiments, the apparatus comprises a residual differential pulsecode modulation decoder (1130); and the controller is responsive to thedetection of the control flag to enable or disable the residualdifferential pulse code modulation decoder.

In embodiments, the apparatus comprises a sample-based angularintra-prediction decoder (1120); and the controller is responsive to thedetection of the control flag to enable or disable the sample-basedangular intra-prediction decoder.

FIG. 22 schematically illustrates a part of an encoder.

A controller such as the controller 345, which is responsive to acurrent lossy or lossless mode, determines whether or not to enableSAIP, RDPCM and edge filtering, for example by means of trial encodingsor other analysis. The controller 345 passes information to a flaggenerator 1200 which generates a control flag (in particular, theintra_RDPCM_enable flag discussed above) for insertion (for example, bythe controller or by the flag generator) into an output data stream1215. The controller 345 also controls operation of an edge filter 1220,an SAIP encoder 1230, an RDPCM encoder 1240 and possibly one or moreother encoders 1250, all to act upon an input video signal 1260. Inparticular, selected encoder generates an encoded signal 1210 which,according to the control and enablement of the controller 345, is actedupon by the edge filter 1220 to generate the output data stream 1215.

FIG. 22 therefore provides an example of a video data encoding apparatusoperable to generate a control flag for associating with at least a partof an encoded image, in which: in a lossless mode of operation, a firstcontrol flag state enables sample-based angular intra-prediction butdisables edge filtering of prediction samples, and a second control flagstate disables sample-based angular intra prediction but enables edgefiltering of prediction samples; and in a lossy mode of operation, thefirst control flag state enables residual differential pulse codemodulation coding and enables edge filtering of prediction samples, andthe second control flag state disables residual differential pulse codemodulation coding but enables edge filtering of prediction samples.

In embodiments, the apparatus comprises an edge filter (1220); and acontroller (345) responsive to the detection of the control flag toenable or disable the edge filter.

In embodiments, the apparatus comprises a residual differential pulsecode modulation encoder (1240): and in which the controller isresponsive to the detection of the control flag to enable or disable theresidual differential pulse code modulation encoder.

In embodiments, the apparatus comprises a sample-based angularintra-prediction encoder (1230); and the controller is responsive to thedetection of the control flag to enable or disable the sample-basedangular intra-prediction encoder.

FIGS. 1 to 4E provide video processing, capture, storage and/ortransmission apparatus comprising such apparatus.

Data Signals

It will be appreciated that data signals generated by the variants ofcoding apparatus discussed above, and storage or transmission mediacarrying such signals, are considered to represent embodiments of thepresent disclosure.

In so far as embodiments of the disclosure have been described as beingimplemented, at least in part, by software-controlled data processingapparatus, it will be appreciated that a non-transitory machine-readablemedium carrying such software, such as an optical disk, a magnetic disk,semiconductor memory or the like, is also considered to represent anembodiment of the present disclosure.

It will be apparent that numerous modifications and variations of thepresent disclosure are possible in light of the above teachings. It istherefore to be understood that within the scope of the appended claims,the technology may be practiced otherwise than as specifically describedherein.

All of the techniques described above have application in encodingapparatus and methods, and/or in decoding apparatus and methods.Techniques discussed In relation to encoding have corresponding featureson the decoding side. Techniques discussed in relation to decoding haveapplication in the decoding path of the encoder. Accordingly, it will beappreciated that the description above of encoding techniques andapparatus, which techniques and apparatus include a decoding path, arealso relevant as examples of decoding techniques and apparatus.

1. A video data decoding apparatus comprising: detector circuitry configured to detect a control flag associated with at least a part of an encoded image for decoding, in which: in a lossless mode of operation, a first control flag state enables sample-based angular intra-prediction but disables edge filtering of prediction samples, and a second control flag state disables sample-based angular intra prediction but enables edge filtering of prediction samples; and in a lossy mode of operation, the first control flag state enables residual differential pulse code modulation coding and enables edge filtering of prediction samples, and the second control flag state disables residual differential pulse code modulation coding but enables edge filtering of prediction samples. 